In most industrial fields, it is desirable to have high-performance insulation systems to ensure that fluids being conveyed in pipework are maintained at constant temperature so that transfers between pieces of equipment can be made possible over considerable distances, e.g. as much as several hundreds of meters, or even several kilometers. Such distances are commonplace in industries such as oil refineries, liquefied natural gas installations (at −165° C.), and undersea oil fields that extend over several tens of kilometers. Such oil fields are being developed in depths of water that are becoming ever deeper, and can be at depths considerably greater than 3000 meters (m).
Numerous systems have been developed for reaching a high level of thermal performance, and specific versions have been developed to accommodate great depths as appropriately as possible, i.e. to be capable of withstanding pressure at the bottom of the sea. The highest performance technologies that have been developed for achieving this objective are so-called pipe-in-pipe (PiP) technologies in which an inner pipe conveys the fluid, and an outer pipe disposed coaxially around the inner pipe comes into contact with the surrounding medium, i.e. water. The annular space between the two pipes can be filled with lagging material, or it can be evacuated so as to be free of gas.
In this type of pipe, the annular space, whether or not filled with lagging material, is generally at an absolute pressure that is lower than atmospheric pressure, and it might be completely evacuated, so to a first approximation, the inner pipe can be considered as radially withstanding the bursting pressure due to the internal fluid, while the outer pipe withstands implosion created by the hydrostatic pressure (ρgh) at the sea bottom, which pressure is about 1 megapascal (MPa) per 100 m of depth of water, i.e. 30 MPa at a depth of 3000 m. The axial effect due to pressure, referred to as the “bottom” effect, acts on the circular section of the pipe and parallel to the axis of said pipe, and is shared, to a first approximation, by both pipes (since they are connected together at their ends), pro rata the respective sections of their materials, generally steel.
For installations for use at great depth, undersea pipes and undersea coaxial pipe assemblies are assembled on land to constitute elements having a unit length of the order of 20 m to 100 m, depending on the support capacity of the laying system. They are then transported in this configuration out to sea on a pipe-laying ship. During laying, the unit lengths of the various coaxial pipe assembly elements are connected to one another on board the ship progressively as laying proceeds. It is therefore important to be able to integrate making the connections in the process for constructing the pipe and laying it on the sea bottom, while slowing the process down as little as possible so that it can be performed quickly and easily.
While laying a conventional PiP in great depth, by way of comparison or as described in this patent, said PiP is subjected to bending, mainly in its bottom portion close to the sea bed. Bending is at a maximum at the point of contact with the sea bed since the radius of curvature decreases from the surface down to the point of contact with the sea bed where it is at its minimum, with the PiP thereafter resting substantially horizontally on the bottom of the sea and presenting a radius of curvature that is infinite. The bending that occurs during laying creates high levels of stress in each of the tubes of the PiP and in the connection zone between two successive lengths of PiP.
For this purpose, use is made of junction parts or connection parts that are made of forged steel and that are assembled to the ends of said coaxial pipe assembly elements for joining together. The junction part at the downstream end of a first coaxial pipe assembly element that has not yet been joined is connected to the junction part at the upstream free end of a second coaxial pipe assembly element that has already been joined at its downstream end.
Patent GB 2 396 196 describes junction parts serving to allow thermal expansion of the inner pipe that is subjected to the temperature of the fluid, generally 60° C. to 100° C., relative to the outer envelope which remains at the temperature of the sea bottom, generally 3° C. to 5° C., and to do this it creates a discontinuity in the inner pipe and a connection between the inner pipe and the outer pipe via a radial wall that is relatively fine and deformable. The junction part described in GB 2 396 196 thus does not enable a rigid connection to be made between the inner pipe and the outer envelope at the ends and suitable for properly transferring traction, compression, and bending stresses between said inner pipe and the outer envelope, and also enabling fatigue phenomena to be withstood during the laying operations and throughout the lifetime of the PiP, particularly with bottom-surface connections that are subjected to the effects of swell and of current, and where said lifetime can exceed 25 or 30 years.
Patents GB-2 161 565 and GB-2 191 842 describe a PiP and its method of assembly, and also two methods of making forged connection or junction parts, the first patent GB-2 161 565 describing a one-piece forging, and the second patent GB-2 191 842 describing a two-piece forging, with the junction between the two pieces of each of the two junction parts being provided by a screw thread, said thread being coated in adhesive to provide sealing.
In both examples, the forging has two circularly symmetrical branches comprising an outer branch and an inner branch defining an annular space, i.e. branches forming a fork with free cylindrical ends that are assembled to the cylindrical ends of the outer and inner pipes, respectively.
Nevertheless, in both embodiments, shortcomings are to be found in the mechanical reliability of the connection between unit lengths of coaxial pipe assemblies fitted at their ends with such junction or connection parts.
One of the shortcomings of the junction forgings proposed in those prior patents lies in the connection zones of said junction parts, since the diameter of the parts is reduced and corresponds substantially to the diameter of the inner pipe. As a result there is a very significant change in the second moment of area of the cross-section of the PiP between the main or intermediate zone of said PiP and said end or connection zone between two of said unit lengths of PiP, which leads to a point of weakness being created at each of these welded connections between two forgings, the zone of said welding then being particularly sensitive to fatigue phenomena, both during laying and during the lifetime of the pipe.
To avoid having such a zone of weakness and to conserve a substantially constant second moment of area for the cross-section, it is possible to increase the wall thickness of the forging over the entire zone situated between the solid portion of said forging and the chamfered zone where welding is performed. However it is then necessary substantially to double said thickness. For pipes of large diameter that are to be laid at great depths, welding becomes problematic because of the very great thickness of steel, since said thickness can be as great as 40 millimeters (mm) to 50 mm, thus requiring welding techniques that are very difficult to perform, and indeed in some circumstances practically impossible to perform without including defects, given the dynamic effects that can be applied to the mass of molten steel while at sea. In addition, since said welding is performed on board pipe-laying ships, which ships present extremely high hourly costs, the cost of an installation becomes prohibitive, and the risks of failure are considerable because of the complexity of said on-site welding operations.
It is then preferable to use the method described in patent FR-2 751 721 which consists in a technique for making the ends of a PiP combined with a technique for reinforcing the connection zone between two unit lengths of PiP by means of a sliding sleeve that presents small clearance relative to the outer envelope, said sliding sleeve being secured to said outer envelope by adhesive. That disposition serves to increase the second moment of area of the cross-section locally so as to limit stresses in the connection zone between two unit lengths of PiP, but it requires several mechanical parts to be manufactured that are complicated to assemble, and it requires connection operations that are relatively difficult to implement. In addition, the proposed adhesive remains subject to creep and deteriorates over the thermal cycling to which pipes are subjected over a lifetime of 20 to 30 years. Finally, that type of adhesive cannot be made reliable for bottom-surface connections, since the dynamic effects of swell and current on the suspended pipe between the sea bottom and the floating support rapidly degrade the plane of adhesive bonding, leading rapidly to excessive fatigue in the PiP connection zones.